The invention relates to a security paper with luminescing authenticity features as well as to a process for testing such a security paper.
Under the term "security paper" are here to be understood bank notes, check forms, shares and stamps as well as passes, credit cards, check cards, passports, air tickets and other certificates and documents.
In order to prevent forgeries and counterfeiting, it has been attempted for a long time to construct security papers which ought to be protected such that unauthorised persons cannot undetectably change or reproduce them.
Accordingly in the past particularly those security methods have recommended themselves the features of which on the one hand can be tested for authenticity umambiguously by anyone without technical assistance and without particular technical knowledge, but which for the manufacture of the feature make necessary such a high degree of a workman's technical skill that the forger is not in the position professionally subsequently to carry out the manufacturing process.
Particularly in the case of bank notes providing these with genuine water marks and with security threads has proved of value, since these can only be undertaken during paper manufacture by means of expensive equipment. Security features of comparable value are extremely fine, and accordingly technically very demanding, steel gravure patterns.
For some time a strong trend to automation in international money transactions has been evident. Accordingly it has emerged that the authenticity features used up till now are not suited to the same degree for automatic testing. Because the checking machinery does not look at the whole of the security paper presented for testing, it is easier to deceive by imitations than the human senses for which these features were designed. Thereby it becomes necessary additionally to the noted visually testable authenticity features to create further ones which can be detected by automatic testers with comparable security.
Printed forgeries accordingly become more difficult to make if the features provided for an automatic testing cannot be registered by the human senses.
In the meantime in the patent literature a series of optical, electrical and magnetic features have been suggested for rendering security papers secure and suited to machine reading. These authenticity characteristics are suitable for testing in devices such as e.g. cash dispensers; these features cannot, however, be unnoticed by the customer and checked without this being seen at the bank counter and in comparable situations.
In the case of security papers with magnetic security threads one is forced, as overall with magnetically effective features, to position the document exactly in the checking apparatus and to make on both sides close contact with the magnetic field detector such as e.g. a coil, a magnetic head, a sound head, a field plate or the like.
For the same reasons electrically conducting inserts or imprints are excluded as a feature for inconspicuous and rapid checking. Additionally generally a stroke with a pencil serves to imitate such a feature.
Security papers with optical authenticity features have already become known in the automatic testing of which neither exact positioning nor close contact with the testing apparatus is necessary.
In a first group of such features the local absorption at wavelengths in the infrared or the ultraviolet spectral regions is tested; the papers are for this purpose provided by suitable means already during their manufacture with transmission patterns. If one does not wish to take on the disadvantage of a large surface print into the costs then in the examination of such security papers one must carry out a difficult pattern recognition. Accordingly the range of application is already much restricted.
A known marking of this type can also be imitated with substances which are likewise available in commerce.
In a second group of optical features the fluorescence emission of characteristic materials is used for giving authenticity checking.
Rendering security papers secure against forgery by means of luminescing substances has already been known for a long time. Already in German Patent Specification No. 449133 from the year 1925 and German Patent Specification No. 497039 from the year 1926 the introduction of luminescing substances into security papers is described, wherein the luminophores used therein can be excited with ultraviolet or other invisible rays and emit in the visible region.
In U.S. Pat. Nos. 3,473,027 and 3,525,698 luminophores and their use as coding dyestuffs on the basis of host lattices doped with rare earth metals which if appropriate can be co-activated, are described, in which the excitation takes place in the UV-region and in the short wave visible region and the emissions in the near IR-region find use for broadening the usable spectral range.
In German Offenlegungsschrift No. 2547768 co-doped rare earth metal luminophores are described which are excited in the IR-region and emit in the visible region.
The use of luminophores for rendering data carriers secure is further described in German Offenlegungsschrift No. 1599011 as well as in German Offenlegungsschrift No. 2903073, wherein the luminous materials theredescribed are stimulated in the IR-region.
In the patent literature and the scientific literature a very great number of different rare earth metal luminophores are described which as single crystals are suitable for solid body lasers or for other uses. For example attention can be directed to U.S. Pat. Nos. 3,447,851 and 3,480,877 in which crystals with garnet structure are described for laser techniques, but the rendering of security papers secure with luminophores is not mentioned.
From the state of the art concerning rendering security papers secure with luminescing substances it can be concluded that the excitation of the luminophore preferably takes place in the UV- or in the IR-region while the emission is either desired or not seen to be damaging in the visible spectrum (VIS).
All luminescence materials noted in the literature in connection with rendering security papers secure also have at least additionally emission in the VIS. Thereby the marking becomes recognizable on corresponding excitation; further all IR-luminophores known in connection with rendering security papers secure emit in a spectral region which can be investigated with commercially normal image converters.
The luminophores are in the case of single layer security papers worked in as paper additions, paper inserts, for example as mottling fibres or security threads, or printing inks.
It has emerged that in rendering security papers secure using rare earth metal luminophores difficulties arise on account of the properties of the same which are described in the following. In the more recent publications, in most cases "data cards" are described, i.e. generally multi-layer security papers, in which these difficulties can be avoided.
One of the difficulties in rendering security papers secure, particularly in rendering security papers secure using printing inks using rare earth metal luminophores, arises from their particle size. In the publications already noted above namely U.S. Pat. No. 3,473,027 and German Offenlegungsschrift No. 2547768 particle sizes of a few .mu.m upwards are noted. For normal printing pigments, however, particle sizes below 1 .mu.m are necessary. Normal previously used rare earth metal luminophores no longer show sufficient effectiveness below a certain particle size, so that on being comminuted the luminescence intensity is substantially lost. They must accordingly be introduced in large quantities; this gives rise to high costs and often leads to insoluble technological problems, since for this the limit of loading the printing ink with additional materials has to be exceeded.
For avoiding these difficulties in relation to particle size, partially soluble organic rare earth metal luminophores are described which, however, because of their nature do not have the solvent fastness necessary for bank note printing.
In rendering security papers secure up till now particular value has been laid on the fact that on excitation in the UV-region or IR-region luminescence arose in the visible region or in the near IR-region easily accessible with commercially available image converters. In automatic authenticity checking of securing papers however it constitutes an additional security factor if the rendering secure is not visible or it is not possible to make this visible with normal means.
In German Offenlegungsschrift No. 15 99 011 for masking inscriptions covering with a foil has already been proposed. Apart from the fact that the foil itself is visible and thereby directs attention particularly to the place of the inscription, use of foils in the case of bank notes and similar security papers is not practicable.